How Does Engineering Permitting Work in Unincorporated King County, WA?

The Critical Areas Ordinance is the first thing an engineer checks on a King County parcel, because it can override everything else on the site plan. Wetlands and streams carry buffers measured in tens to hundreds of feet depending on the resource rating; steep slopes and landslide hazard areas restrict grading and foundation placement; erosion hazard areas along the bluffs and ravines limit disturbance. The county's review asks for delineation, classification, and a mitigation sequence — avoid, minimize, then compensate — documented by qualified specialists. For the structural and civil engineer, the practical consequence is that the building envelope is smaller than the lot lines suggest. A five-acre rural parcel can lose half its usable area to buffers and setbacks, and the remaining envelope must still accommodate the septic drainfield reserve area, the stormwater facility, fire turnaround, and access. The geotechnical investigation has to speak to slope stability where landslide hazards are mapped, and foundations near steep slopes get setback and drainage conditions. Projects that start with a critical areas reconnaissance and a pre-application meeting with the county move; projects that discover the wetland at 90 percent design do not.

On an unincorporated lot, the civil engineer is effectively the utility department. Stormwater must be managed on site per the county manual — roof and driveway runoff collected, treated, and dispersed or detained so post-development flows match the manual's targets. Infiltration is preferred where soils allow, which means the geotechnical boring program should include infiltration testing early; discovering tight glacial till at the end of design forces a redesign to detention. The county reviews the drainage report, calculations, and details as a core part of the permit, not an accessory. Wastewater is the parallel track: most unincorporated parcels are on septic, and the septic design goes through Public Health with its own soil, sizing, and reserve-area requirements. The drainfield and its 100-percent reserve area consume prime flat ground — the same ground the stormwater facility and the building want. Coordinating these three — building, septic, stormwater — on a constrained rural lot is the central civil puzzle of a King County project, and it is why the site plan should be drawn around the constraints rather than fitted to them afterward.